Oxide are recognized as an important type of devices for next generation energy storage due to their high power densities and excellent cycling stability. They can be used as power supplies where fast and high power delivery and uptake are needed such as emergency doors on air planes. They can also provide transient power assistance to batteries and fuel cells in electric vehicles. There are two types of supercapacitors depending on energy storage mechanisms, namely electrical double layer (EDL) capacitors based on ion adsorption and pseudocapacitors based on electrochemical redox reactions.